Lutetia will be the largest asteroid visited by a spacecraft, at least until 2011. The current record is held by Mathilde, but Lutetia's twice as big. It'll be the first asteroid of spectral class M to be visited, although it doesn't appear to be metallic... it should be a fun one.

A quick recap of facts about Lutetia from "Robotic Exploration of the Solar System - Part 3":

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Almost 100 km in size, Lutetia was discovered on 15 November 1852 by Hermann M.S. Goldschmidt in Paris and dedicated to the town itself, which was known to the Romans as Lutetia Parisiorum (Lutetia of the Parisii tribe).

QUOTE

On penetrating the asteroid belt for the second time, the vehicle will pass Lutetia on 10 July 2010 at a range of about 3,055 km, traveling at a relative speed of 15 km/s. The data from the IRAS infrared astronomy satellite enabled the average diameter of this object to be roughly estimated during the 1980s at 95.5 km, but a number of light curves obtained since, most recently by the Hubble Space Telescope, have enabled astronomers to determine that it is asymmetric and about 132 x 101 x 93 km across, and to rotate in a little over 8 hours. Observations at a range of ‘look angles’ even revealed variations which hinted at the presence of one or more large craters in the northern hemisphere. Lutetia is therefore one of the best-characterized asteroids to receive a spacecraft inspection. However, despite a large number of spectroscopic studies, its taxonomic class was debated. For a long time it was listed as an M-class object that spectrally resembled some iron-rich meteorites. As yet, no such body has been visited. But this classification was not confirmed by infrared observations by the European Southern Observatory, a telescope in Hawaii and the Spitzer Space Telescope, nor by ultraviolet observations by the Hubble Space Telescope. Hence Lutetia is now regarded as an anomalous C-class object, reflecting more light than other members of the class, possibly implying that its surface is ‘unweathered’. In fact, Lutetia appears similar to some metal-rich carbonaceous chondrite meteorites. Rosetta made observations of Lutetia in January 2007, just before the Mars flyby, in order to assist in characterizing its light curve, rotation period and spin axis.

It would have to be faint indeed. If confirmed, that would be the smallest known body with an atmosphere, correct?I can't think of anything off the top of my head smaller then Enceladus that has one....

"Exosphere" in this context seems like a pretty broad application of the term, though. Any solid body is going to emit a certain amount of gas in a vacuum, however exceedingly small that might be.

It's a smart & interesting experiment nevertheless. There appears to be a continuum of possible volatile content levels for members of the Belt's general asteroid population as well as an increasing number of objects that seem to be intermediate between the traditional categories of "comet" & "asteroid". Measuring the emissions of Luetia will provide some very valuable baseline data that directly pertains to all that.

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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.

I think a good rule of thumb would be: it's an atmosphere if the body it surrounds is more or less spherical. That's a good a reference point as any right? Lutetia is excluded, but Ceres and maybe Vesta wouldn't be.

I think a good rule of thumb would be: it's an atmosphere if the body it surrounds is more or less spherical.

Somebody please correct me if I'm wrong, but wouldn't a gravitationally-bound gaseous envelope (yep, intentionally avoiding the "A" word, here) be spherically distributed around a body's center of mass regardless of its gross shape? In fact, wouldn't it follow the contours of its Hill sphere?

There might be pronounced local variations if there were very significant asymmetries in a body's internal mass distribution, but they'd have to be really big mascons, almost certainly much, much larger than any you'd expect to find on an asteroid.

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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.

I think you're right, nprev. We don't see the atmospheric pressure at the top of Mt Everest being any higher than at any other equal altitude on Earth (though this may be a bad example due to the high mass of the planet).

Also, just to throw this in, Mercury has a non-spherical exosphere due to stellar wind effects.

I think you're right, nprev. We don't see the atmospheric pressure at the top of Mt Everest being any higher than at any other equal altitude on Earth (though this may be a bad example due to the high mass of the planet).

The air pressure tends to be substantially lower at the poles, even at the same altitude. For example, the summit of Denali (at about 6.2 km) has an air pressure equivalent to that at 6.9km altitude in the Himalayas (say Everest).

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